In recent years, the communications industry has shown a growing interest in various types of wireless communications systems for communicating voice and/or data between numerous remote sites and a central location. It is well recognized that the use of a dedicated telephone facility for a conventional telephone system is not a convenient or economical option for all communications applications. For example, individuals who must be away from their base of operations have a need to conveniently and efficiently communicate with their base. Likewise, for many industrial applications, a central data collection site has a need for acquiring information from a variety of remotely located monitoring devices that collect data about the operation or performance of equipment. To overcome the limitations of the conventional telephone system, a two-way wireless communications link is often necessary to permit a response to a communication initiated from another location. In an attempt to solve the problem of supplying a response to an initial communication, the industry has offered various wireless communications systems, including two-way radios, mobile radiotelephones, and paging systems.
A conventional radio communications system uses a base station transceiver located at a site for favorable radio propagation and a set of transceivers typically located in vehicles, such as for police or trucking dispatching systems, or at remote equipment sites that communicate data in response to a command signal from the base station transceiver. Most radio communications systems are useful for conducting communications between short distances, such as within the boundaries of a town or city, via a very high frequency (VHF) radio link.
Although conventional radio communications systems are useful for specialized communications applications, two-way radios are not widely accepted for general purpose communications. The use of two-way radios is generally regulated by the Federal Communications Commission (FCC) and the allocated frequency spectrum is relatively limited. Furthermore, the quality of communications is subject to the propagation conditions between communications sites. Two-way radio equipment is generally large and heavy and, as a result, is unlikely to be carried by a user at all times or to be installed in small or existing equipment.
Another form of two-way communications is a cellular mobile radiotelephone (CMR) system, which is connected to the extensive public switched telephone network (PSTN) and permits communications between a mobile radiotelephone user and anyone with a conventional telephone (or another radiotelephone). Typical CMR systems are characterized by dividing a radio coverage area into smaller coverage areas or "cell" using low power transmitters and coverage-restricted receivers. As shown in U.S. Pat. Nos. 3,906,166 and 4,268,722, the limited coverage area enables the radio channels used in one cell to be reused in another cell. As a cellular mobile radiotelephone within one cell moves across the boundary of the cell and into an adjacent cell, control circuitry associated with the cells detects that the signal strength of the radiotelephone in the just-entered cell is stronger, and communications with the radiotelephone are "handed-off" to the just-entered cell. Thus, a CMR system can supply two-way communications for an array of cells, thereby supplying communications for a much wider area than conventional two-way radios.
Conventional radiotelephones generally offer both voice and data communications capabilities and, accordingly, the radiotelephone service is generally supplied at a cost that is commensurate with the combined voice and data services. Nevertheless, this combination of both voice and data communications may exceed a user's requirement for communicating by only voice or data. In addition, real-time voice or data communications is not always desirable by a user who wishes only to receive a message without having a current activity disturbed. Similar to two-way radios, the frequency spectrum for the CMR system radio channels, particularly voice channels, is a limited resource.
Paging systems include pagers, which are miniature receivers, and at least one paging terminal having a transmitter that covers a selected geographic area containing numerous pagers. A pager is generally tuned to a particular tone or data bit-modulated radio frequency that is shared with many other pagers. A particular sequence of tones or data bits is used as an address or the identification of a selected pager. Reception of the particular sequence activates an acoustic, tactile, or visible alert, thereby indicating that a call has been placed to the paging terminal from a telephone connected to the PSTN and generally communicated via the transmitter to the pager. Depending upon the complexity of the pager, the paging device may receive a voice or a data message following the alert, or the alert alone may simply indicate to the user that a call was made and a prearranged action should be taken. This type of pager forms a one-way messaging system because it does not permit the party initiating the page to know if the page has actually been received by the addressed pager.
To respond to a page received via the conventional one-way paging system, the paged party typically must find an available conventional telephone and initiate a telephone call to the party that initiated the page. Alternatively, the paged party can place a responsive call by use of a mobile radiotelephone, if available. In recognition of this problem of separate pager and radiotelephone devices, U.S. Pat. No. 5,148,473 describes a pager and radiotelephone apparatus having the combination of a radio pager section and a cellular radiotelephone section. To answer a page, the user simply places a call via the CMR system. The pager also allows the user to screen incoming call to determine which of the received messages to return, thereby eliminating the CMR system service costs associated with unwanted calls. However, any acknowledgment of a received page by this apparatus requires use of the relatively limited spectrum for voice channels of the CMR system.
Paging systems having two-way communications capability for receiving a page and transmitting an acknowledge receipt of a page are also known. One such system enables a pager having acknowledgment capability to immediately communicate a response to the calling party after receiving the initial page contact. The response may be a voice, numeric, or alphanumeric message informing the originator of the page communication that the message was successfully received by the page, and may further offer a reply to the received message. This page acknowledgment system generally includes a single central transmitter covering a certain geographic area and one or more acknowledgment system receivers for receiving acknowledgment signals from the pagers. The acknowledgment system receivers can communicate acknowledgment messages to the central transmitter via either a wire or wireless link. The number of acknowledgment system receivers is a function of the transmitting power of the acknowledgment transmitter associated with each of the pagers. The transmitter power of pagers is generally low, typically less than one watt, based upon the requirement for portability of the pager and the necessity to minimize the size of the battery for powering the transmitter. Such paging acknowledgment systems are most useful for local area "on-site" communications systems in view of the power limitations of the transmitters associated with the pagers.
Present paging acknowledgment systems require a large capital investment for the purchase and installation of equipment to implement a two-way communications system that covers a wide communications area because the number of acknowledgment receivers (and transmitters) in a paging acknowledgment system is a function of the limited transmitting power of the transmitter associated with each of the pagers. Although the paging industry is interested in supplying a page acknowledgment to subscribers of paging services, the cost of implementing the paging acknowledgment system has discouraged many service providers from installing such paging acknowledgment systems.
Accordingly, there is a need to overcome the limitations of the prior art by adapting an existing communications network to communicate data between a central location and numerous remote sites. There is a further need to adapt an existing communications network to deliver the acknowledgment of a message. This new use of an existing communications system should have a minimum impact upon present communications carried by the system. The present invention adapts the existing architecture of a CMR system in a efficient and cost-effective manner to support data communications via the CMR system, including the collection and reporting of data recorded at remote sites and the acknowledgment of a page message.